Air cond dont work sometimes...

[EuJax]

Hi guys, of late my E30 had some issue on the air cond. Sometimes when i turn it on, there is air but not cold air. Its basically just fan. I understand there is a switch that controls this and its hidden in the glove box area so sometimes i knock it on the glove box, it came back. Really need to rectify this problem. Can anyone help please? Thanks. Regards